Conventional apparatus for automatically mounting electronic components in position on the surface of printed boards have a suction head assembly provided with a suction nozzle for attracting thereto the electronic component by a vacuum. The suction head assembly is mounted on a reciprocating device which is controllable to move in the directions of X-axis, Y-axis and Z-axis.
When the above apparatus is used for surface mounting, the suction head assembly is first moved to a component feed portion of an electronic component feeder, the suction nozzle is caused to attract the electronic component thereto by a vacuum, and the component is thereafter moved to a position above a printed board, whereupon the assembly is lowered to place the component in position on the surface of the board.
When the component is brought into contact with the printed board, the suction head assembly is further driven downward, whereby the component is pressed against a bonding layer or solder paste layer on the board surface and fixed or temporarily fixed to the board.
The suction head assembly is thereafter returned to above the component feeder, followed by the subsequent cycle of surface mounting operation.
With the conventional surface mounting apparatus, the upward and downward movement of the suction head assembly is effected by operating a cam mechanism. The cam curve of the cam mechanism is so designed that an excessive impact force or pressure will not act on electronic components when the suction nozzle is pressed against the component on the feeder and also when the attracted component is pressed against the printed board.
In recent years, however, a wide variety of electronic components are introduced into use which differ variously in configuration, size (thickness) and material. These components include, for example, delicate parts such as IC (LSI) bear chips. The electronic components to be mounted by the surface mounting apparatus therefore differ greatly in mechanical properties with the type of components.
Accordingly, in mounting different types of electronic components by the surface mounting apparatus, the pressure to be exerted on the component when the component is held by suction and when it is mounted on the surface must be controlled to a suitable value for each of the different types.
The suction head assemblies already proposed for use in surface mounting apparatus include, for example, those which are driven by a linear motor (Unexamined Japanese Patent Publications SHO 60-66500 and SHO 63-232496), those which are driven by a compression spring (Unexamined Japanese Patent Publication SHO 63-22292) and those wherein electropneumatic regulator is utilized (Unexamined Japanese Patent Publication HEI 1-246899).
However, the apparatus wherein the linear motor is used for driving has the problem that the apparatus is complex in construction and has a great weight. With the apparatus wherein the compression spring or electropneumatic regulator is used, it is difficult to accurately control the pressure or impact force to be actually exerted on the electronic component.
Further in the case of electronic components which are not resistant to impact forces, the force of impact to be produced by the striking contact of the component with the printed board must be limited to not greater than a specified value.
Nevertheless, the value of impact force permissible for components is not definite for all components, whereas the suction head assembly is conventionally lowered at a suitable speed which is empirically determined. This entails the problem that an excessive impact force is likely to act on some components to shorten the life of the component or break the component.
The lowering speed, if set to a very low value, results in a poor work efficiency and is not practical.